At present, a discharge type fluorescent lamp and an incandescent bulb used as the illumination device involve problems that a harmful substance such as mercury is contained, and life span is short. However, in recent years, a high luminescence LED emitting light of near ultraviolet/ultraviolet to blue color has been developed in sequence, and the white LED illumination for the practical application of the next generation has been actively studied and developed, in which the white light is created by mixing the light of the near ultraviolet/ultraviolet to blue color generated from the LED and the light generated from the phosphor having an excitation band in a wavelength region thereof. When the white LED illumination is put to practical use, since efficiency of converting electric energy into light is improved, less heat is generated and it is constituted of the LED and a phosphor, the white LED has advantages of good life span without burn-out of a filament like a conventional incandescent bulb and the harmful substance such as mercury is not contained, and miniaturization of the illumination device is realized, thus realizing an ideal illumination device.
Two systems are proposed as the system of the LED illumination. One of them is a multi-chip type system which creates white color by using three primary color LEDs such as high luminance red LED, green LED, and blue LED, and the other one is one-chip type system which creates white color by combining the high luminance LED emitting light of near ultraviolet/ultraviolet to blue color and the phosphor excited by the light of the near ultraviolet/ultraviolet to blue color emitted from this LED.
In a one-chip scheme in particular, a method is generally used in which a Y3Al5O12:Ce yellow phosphor having a garnet structure is combined with a blue LED; La, Tb, Gd, or another rare-earth element that has a large atomic radius in the same manner as Y is substituted in place of or added to the Y sites of this yellow phosphor Y3Al5O12:Ce; and B, Ga, or another trivalent element having a small atomic radius in the same manner as Al is substituted in place of or added to the Al sites, whereby various emission colors in the range from green color to reddish yellow color can be obtained while maintaining a garnet structure. For this reason, combinations with the light emitted from the blue LED can be used and various white lights having different color temperatures can be obtained.
However, the emission wavelength or emission color of a YAG:Ce phosphor can be changed by substituting or adding various elements, but emission efficiency is reduced by substituting elements, and the emission intensity is dramatically reduced at a temperature of 100° C. or higher. For this reason, there is a problem in that the balance between the emission color of the phosphor and the emission element is degraded and the tone of the white color is changed. On the other hand, a light emitting scheme that uses UV or near UV LED has a problem in that the emission intensity is dramatically reduced at temperatures of 100° C. or higher in the same manner as in a case in which ZnS:Cu, Al; (Sr, Ca)GaS:Eu; or another sulfurous phosphor having good light-emission characteristics is used as the phosphor.
In order to solve such temperature-induced degradation problems, demand has increased for novel phosphors that have a flat, high-efficiency excitation band in the range of near ultraviolet/ultraviolet to blue light, and that have excellent stability of light emission characteristics in relation to the ambient temperature. For example, much research has been carried out in relation to, e.g., Ca-sialon-based phosphors (described in patent document 1) and other oxynitride phosphors. However, there are problems with Ca-sialon-based phosphors in that the emission efficiency of the phosphor is insufficient in comparison with a YAG:Ce phosphor, and since the half-value width is narrow, adequate brightness and color rendering properties cannot be obtained except at a specific color temperature, a plurality of phosphors must be mixed to obtain an emission device that has excellent color rendering properties, and the overall emission characteristics as an emission device are thereby reduced.
There has also been proposed a (Ca, Sr)2Si5N8:Ce yellow/green color phosphor (see patent document 6), which is a nitride phosphor of the same system as the phosphor described in patent document 1. The (Ca, Sr)2Si5N8:Ce yellow/green color phosphor has a problem in that the emission characteristics are poor and stability is reduced due to heat, but non-patent document 1 describes a solution to this problem in which Li or Na is added in a small amount to the phosphor matrix, whereby the emission characteristics are improved.
Disclosed in patent document 2 is an Sr—Al—Si—O—N phosphor that is different from the Ca-sialon-based phosphor, i.e., SrSiAl2O3N2:Ce, SrSiAl2O3N2:Eu, and Sr2Si4AlON7:Eu. However, these have poor emission efficiency, and are blue phosphors having a wavelength of 450 nm to 500 nm or red phosphors having a wavelength of 630 nm to 640 nm. A phosphor having good emission efficiency and an emission color from blue to orange in which the emission wavelength is 500 to 620 mm cannot be obtained.
In order to solve these problems, the present inventors developed a phosphor having a novel structure based on Sr—Al—Si—O—N, as described in patent document 3, and have proposed a phosphor that emits a good green to yellow color even from excitation light in the range of blue or near ultraviolet/ultraviolet, and have furthermore proposed in patent document 4 an Sr—Al—Si—O—N-based phosphor provided with good temperature characteristics and emission efficiency at high temperatures by adjusting the addition amount of Al. Also, an emission device that has good color rendering properties has been proposed in patent document 5 in which a plurality of phosphors comprising the above-described phosphors, a red phosphor, a blue phosphor, and the like is combined with an LED that emits ultraviolet to blue as an excitation light.
[Patent document 1] Japanese Laid Open Patent Application No. 2002-363554
[Patent document 2] Japanese Laid Open Patent Application No. 2003-206481
[Patent document 3] Japanese Patent Application No. 2005-061627
[Patent document 4] Japanese Patent Application No. 2005-192691
[Patent document 5] Japanese Patent Application No. 2005-075854
[Patent document 6] Japanese Laid Open Patent Application No. 2002-322474
[Non-patent document 1] Journal of Luminescence, 116 (2006), 107 to 116